1. Field of the Invention
The invention relates to creating images by transmitting signals and sensing the effect of objects in the field of view on the signals.
2. Description of the Prior Art
Video cameras have a well known capability to sense visible light and to electronically produce signals that enable a standard television set to display an image. Looking at that image is much the same as looking at the scene directly. It is possible to produce comparable images where visible light methods fail by using other forms of radiating waves. This can be done by beamforming methods where the lens is replaced by arrays of sensors and electronic equipment. This tends to be most useful where the wavelength of the waves is substantially longer than the wavelengths of visible light. The immediate application is in under water acoustic imaging, though there are other applications in medical imaging and radar. In describing such systems, the video camera is the standard of comparison.
Video cameras correctly portray the shape of objects. They provide high resolution to make pictures seem real. They respond immediately to the scene at which they are pointed and they produce pictures at a frame rate suitable for representing moving objects. In deep ocean exploration, under water video cameras are useful if artificial light is provided, but this works only if the camera is close to the object of interest.
For longer range underwater viewing other methods are required. Acoustic imaging systems exist, but these produce images which are much inferior to video camera images. Most existing systems produce images which do not look at all like the scene. Special interpretations skills are needed to understand these images. Also, the equipment capable of achieving high resolution tends to be operationally awkward. The most common underwater imaging systems are side scan sonars. These require a series of exposures to produce an image and the rate that this can be done is satisfactory only for stationary scenes. Forward looking sonars, that have the capability to achieve high resolution, are large and heavy.
There is a clear need for equipment that produces high resolution pictures, directly represents a scene as if viewed by the eye, operates at frame rates suitable for underwater operations, and performs at greater range than an underwater video camera. The sonar engineering community has long been aware of this need but has not, until now, produced equipment comparable to a video camera.
Previous imaging systems, that include coordinated beamforming with transmitting and receiving transducer arrays, are relevant prior art. Transmit beamforming is a process of producing phase related signals that are radiated by transmitting transducers in an array, such that signals from each transducer overlap, with phase relationships that cause concentration of strong signals in a particular direction. Receive beamforming is a process of combining signals that are received by receiving transducers in an array, by adjusting the phase relationships between signals and adding the signals, to cause enhanced sensitivity in a particular direction. The beams produced by these beamforming processes are geometrically described in the same way for both transmitting and receiving systems. The direction of a beam is an angle relative to the array.
Beamforming produces beams which can be used to form images. The beams are used in a system that sends out signals and then senses reflected signals. This is done to individually detect target points in a scene, determine the location of each target point, and enter the detected signal strength, as a level of gray, at the appropriate corresponding position on a display. The detected signal strength is an image signal sample. An image signal is a sequence of such samples. The beams serve to determine the location of the target points because it is known which beams detected that target point.
In optics, a lens is a beamformer that produces an extremely large number of simultaneous beams. For example, 10 million beams can be produced by a one inch lens. A lens usually produces more beams than are needed by the optical system. In optics the resolution cell, or pixel, is often determined by the granularity of the detecting surface. In electronics, it is very challenging to produce enough beams, so the resolution cell is determined by the number of beams.
An important transmitting system method was described in 3,447,125 (5/1969) Peugh. Here was used a transmitting system where a coding system enabled an array beamforming system to transmit many beams simultaneously. The coding system was necessary for simultaneous transmission of many beams where the codes prevented degeneration of the process. The coding system was also essential to localize targets. The special importance of this is that many beams cover a large area as a result of a single burst transmission. This is more important in acoustic systems where it takes significant time for the two way propagation paths. In stationary situations sequential operations can be used to form and scan the direction of beams but this takes a very long time. This ceding method makes rapid image formation possible.
Receive beamforming is a sophisticated technology that has been utilized in conventional forward looking sonar systems for many years.
Simultaneous transmit beamforming and simultaneous receive beamforming are necessary for imaging systems to operate at rapid frame rates such that moving objects can be correctly portrayed or that scenes viewed from a moving vehicle can be correctly portrayed.
The transmit coding method described by 3,447,125 (5/1969) Peugh was included in 3,794,964 (2/1974) Katakura where a comparable beamforming operation was done using an acoustic lens. Here was described an important combination of transmit beams and receive beams where the two sets of beams served to determine position of targets in two orthogonal dimensions. 3,794,964 (2/1974) Katakura created an imaging system using this orthogonal beam technique for medical imaging purposes.
Another combination of transmit beams and receive beams was attempted by 4,456,982 (6/1984) Tournois which also used the method described by 3,447,125 (5/1969) Peugh to create an imaging system where orthogonal transmit and receive beams were used to determine target position. This system used arrays of omnidirectional transducers with beamforming techniques to directly produce the beams. 4,456,982 (6/1984) Tournois rejected the use of a lens. This system used two arrays where, in each array, transducers were arranged along a line. These are called linear arrays.
A linear array produces a beam by the beamforming process defined previously. Far from the array, such a beam can be described as a region between two concentric cones where the line of the array is the common axis and the cones have a common vertex point. The cone angles measured from the axis describe the intended beam. The responsive region fully encircles the axis.
The system described by 4,456,982 (6/1984) Tournois operated to produce many such beams simultaneously, both for transmission and reception. The arrays were arranged perpendicular to each other such that the transmit beams and the receive beams operated as a system to determine the position of target points in two dimensions. One dimension was known by knowing the angle from which signals were received. The other dimension of the target position was determined by the method described in 3,447,125 (5/1969) Peugh. This other dimension was the angle of the transmit beam that covered the target, where that transmit beam was identified by identifying the code of the received signal. For every receive beam there was a set of code channels which represented a set of transmit beams.
The combination of transmit and receive beams can be viewed as two nests of many cones where the cones in each nest have a common axis line and a common vertex point. In a nest, each cone angle is slightly larger than the previous and the cone angles, measured relative to the axis line, range in value from 0 to 180 degrees. As cone angles approach 0 or 180 degrees the angle increment grows larger. The combination is made by arranging the two nests of cones such that their axis lines are perpendicular and their vertex points are the same point. The intersection of these two nests defines many solid angle sectors. These are regions which serve to localize, or resolve, any target in space. Such regions provide positions in two angular dimensions relative to the two arrays. The distance, or range, of a target point from the two arrays is not determined by such a system. A scene to be imaged is typically represented by many target points and their relative positions must be determined to form an image. A target point is a scattering center.
The way these nests of cones interact illustrates problems in producing images with the conventional linear arrays described by 4,456,982 (6/1984) Tournois.
The first problem was the fact that any two orthogonal beams intersect in two directions. This ambiguity could not have been resolved using the linear arrays described by 4,456,982 (6/1984) Tournois and the actual described implementation produced double images. The possibility of other array configurations was mentioned and there are conceivable, though impractical, two dimensional array configurations that would solve this problem. The omnidirectional transducers specified by 4,456,982 (6/1984) Tournois did not allow other, more practical, ways to resolve this ambiguity. A more complicated array system is required where directional transducers contribute to the performance of the array system.
There is another linear array problem that can be visualized in terms of the two nests of cones. Clustered around a boresight direction, that is perpendicular to both cone nest axes, is a set of regions that form an approximately rectangular grid when projected onto a perpendicular planar surface. This cluster area is good for creating an image. At directions far outside this cluster, there is a widening of angle sectors which degrades the resolution. In the diagonal directions, on that planar surface, the intersection of cones begins to form regions that are non-rectangular in shape and the grid becomes large. Assuming that geometric corrections are made, the effect of this is to degrade the image, making it look more granular.
The system produced by 4,456,982 (6/1984) Tournois had the capability to form images over a 180 degree field of view. This included both the desirable and undesirable image areas and provided a wider field of view than usually needed. The simplest way to limit images to a smaller field of view was to just ignore much of the data, so the useful image was formed using only a fraction of the available data points. To improve the resolution in the smaller field of view it was necessary to lengthen the arrays by adding more elements. This improved the resolution over the full 180 degree field of view at a cost of increasing the number of transducers. If the arrays had been simply lengthened using the stone number of transducers then spurious response of the arrays would have caused ghost image problems. The requirement to adequately sample the space was imposed by 4,456,982 (6/1984) Tournois to prevent such undesirable, false image confusion. Conventional array theory dictates the rules for sampling the space. Spatial sampling rules require transducer spacing that does not allow high resolution over a smaller field of view without increasing the number of transducers.
Another significant problem in 4,456,982 (6/1984) Tournois was inadequate means of focusing transmit beams. This is a serious limitation for high resolution systems that use long wavelengths to form images. Operating in the near field means that wavefronts at the array can not be represented as a planar surfaces. It also means that the description of beams, in terms of cones, becomes inexact though it is still accurate enough for much of the interesting operating range. Operating in the near field requires a complicated focusing process for the beamforming operations. Sophisticated means of dealing with this problem for the receive beamforming process are known in sonar engineering. Adequate transmit beam focusing methods are not found in prior art.
3,447,125 (5/1969) Peugh provided for far field focusing only. 3,794,964 (2/1974) Katakura provided the capability to focus at a particular range by use of the lens, where the choice of range was predetermined by the lens and the hardware placement. A flexible capability was needed for underwater operations. 4,456,982 (6/1984) Tournois did not discuss the need for near field focusing capability where it appears that this system was built only far field operations since the need for range dependent focusing was not mentioned. 4,456,982 (6/1984) Tournois described a relatively low resolution system which was less sensitive to near field effects.
For high resolution systems, the necessary focusing system must take into account the different range to different points in the image and focus must be accordingly provided. The beam coding systems of 3,447,125 (5/1969) Peugh, 3,794,964 (2/1974) Katakura, and 4,456,982 (6/1984) Tournois were all based on angle only. These associated a single unique code with each direction. For transmit beams this meant that a single range at which to focus the beams had to be determined before the burst transmission. In the near field this meant that sharp focus existed over a limited range extent. This is the depth of field. The formation of images over an extended field of view, as a result of a single burst transmission, must take into account an additional dimension. The coding system must be based on both range and angle dimensions.
A two dimensional coding system was partly represented in 5,142,649 (8/1992) O'Donnell, where a dynamic transmit beam focusing system took account of both range and angle dimensions. In this imaging system a different method of image formation was used where one array was used for both transmitting and receiving and coordinated operation of two arrays was not involved.
4,855,961 (8/1989) Jaffe et al. described related prior art where a useful coding system is included as part of another kind of system. In this system each transmitter has the capability to form an exclusive beam as a result of its own size and characteristics and each transmitter does not depend on interaction with other transmitters to form a beam. This does not fit the definition of beamforming, used in this document, where radiation from the transducers overlaps and interacts to form a beam. This related invention suggests an unconventional array method where the ghost problems brought about by sparse spatial sampling are suppressed by the directionality of the individual transmitting transducers.
Thus, there is prior art that includes important relevant technologies, but previous imaging systems are limited in capability. The array methods of 4,456,984 (6/1984) Tournois required high density arrays which limited the ability to efficiently form high resolution images over a useful field of view. 4,456,984 (6/1984) Tournois required omnidirectional transducers which also limited ways of achieving adequate performance. The coded beamforming systems of 3,447,125 (5/1969) Peugh, 3,794,964 (2/1974) Katakura, and 4,456,982 (6/1984) Tournois failed to provide for extended depth of field focusing which made near field operations unsatisfactory. The need for a system that correctly portrays the shape of objects, provides high resolution, operates over an appropriate range, and can adequately show moving scenes is not satisfied by prior art systems.
The patents referenced in this document are incorporated by reference. In case of conflict, the present document takes precedence in all respects.
The general object of the present invention is to produce images with array beamforming methods that are comparable with standard television images. The popular video camera is the basis of comparison. An immediate object is to provide a system that will operate compatibly with underwater video cameras to improve underwater exploration systems. A complete underwater exploration system is also an object of this invention.
The present invention is a beamformed television system. Objects of this invention are accomplished with features that are represented in prior art systems, features that are known in sonar engineering practice, and some important new technological developments that replace features of prior art systems. These features are advantages of the beamformed television system.
An object of the beamformed television system is to form images in rapid succession. An advantage is a high speed image formation method based on simultaneous beamforming methods, both in a transmitting system and a receiving system.
An object of the beamformed television system is to provide a large number of resolution cells using an orthogonal beam system where beams are formed in transmitting and receiving systems. An advantage is an electronic beamforming method that is more flexible than the acoustic lens method.
An object of the beamformed television system is to efficiently achieve high resolution over a useful field of view. An advantage is an unconventional array system that enables this objective without requiring the large number of transducers called for by conventional array sampling rules. A related object is the suppression of ghost images that arise with unconventional spatial sampling array methods. An advantage is an interactive array method, which effectively suppresses ghost images. This enable the previous advantageous feature.
Objects of the beamformed television system are to resolve the front to back ambiguity and to provide further attenuation of unwanted signals. Advantages are a complex array system that uses directional transducers which provide the necessary capabilities. Omnidirectional transducers fail to provide the needed discrimination.
An object of the beamformed television system is to produce sharply focused images over an extended field of view in a single exposure operation. An advantage is a beam segment coding system that is based on the two dimensions of range and angle where focusing is achieved over the required number of beam segments. A beam coding system that is based on angle only does not have the flexibility to provide an extended field of view capability.
The described objects of the beamformed television system are necessary to meet the general object of this invention. The associated advantages are the features of this invention that enable it to achieve the objects and, hence, achieve the general object where prior art inventions do not satisfy this need. No prior art system is available that is comparable with a video camera. The intended complete underwater exploration system is not possible without this capability.
Further objects and advantages of the present invention will become apparent from a consideration of the drawings and ensuing description.